1. Field of the Invention
This invention relates to advanced military grade communications jamming systems and, more specifically, to a Method and Apparatus for Automatic Jammer Frequency Control of Surgical Reactive Jammers.
2. Description of Related Art
Modern military grade communication systems today employ short, burst type transmissions that constantly cycle through a secret sequence of frequencies in order to prevent detection and jamming. Such systems are commonly known as frequency hoppers. Typically, these systems (both foreign and domestic) only transmit on a particular frequency for no more than a few milliseconds at the most. This creates a problem for those who want to detect and jam such transmissions as they happen so quickly. Practically, it is not feasible to simply “splash” the radio frequency spectrum with random noise in order to jam such transmissions. The reasons are that it requires an unpractical amount of power to apply sufficient RF energy to wash out all transmissions. In addition, there may be friendly transmissions that should not be jammed. Also, since the duration of the target transmissions is so short, it is not practical to have (for instance) a CPU that is programmed to evaluate signals, make a determination, and then command transmitters to jam. There is simply not enough time to engage the frequency hopping signals before they have moved on to a new frequency.
This invention relates to an enhanced electronic warfare capability over enemy signals and communications; it is an extension and an augmentation to a previously filed U.S. patent application Ser. No. 10/912,976: “System and Method to Autonomously and Selectively Jam Frequency Hopping Signals in Near Real-Time,” also written by this author. That patent will be referenced continually throughout this description.
The jammer device described by patent application Ser. No. 10/912,976 is sometimes referred to in the Electronic Warfare industry as a “wideband reactive jammer”, “surgical follower jammer,” or a “surgical reactive jammer” because it has the ability to quickly find enemy signals and then very rapidly apply jamming energy right on targets so as to jam those enemy communication signals. This is possible because it uses a wideband digital reception technique to instantaneously detect the presence of enemy signal energy. Once the enemy signals are detected, they are then immediately jammed by using fast direct digital synthesizers to output RF energy right on those detected enemy signal frequencies.
One of the tradeoffs of this sophisticated technology, though, is that to enhance jammer reaction speed, absolute frequency resolution is often sacrificed. Thus, the reactive jammers output signals might not be exactly on target, given that the frequency detected was not completely precise. There are a variety of jammer techniques, which have been invented and employed by this author to overcome this shortcoming. One such unique technique is shifting the jammer frequency very quickly, and pseudo-randomly, across the maximum theoretical error deviation. Thus, the target receiver experiences pulsed frequency and amplitude modulated jamming tones that hit the right frequency most of the time. That technique is the topic of a separate patent application. The other technique is the very topic of this patent application, which is hereby coined as Automatic Jammer Frequency Control (AJFC).
What is needed therefore in order to feasibly detect and jam modem fast narrowband transmissions as efficiently as possible is a system that has not only has: 1) The abilities stated in the aforementioned previous patent application Ser. No. 10/912,976, but also 2) The ability to implement AJFC to fine tune the jammer signals in real time.
FIG. 1 is a functional depiction of a preferred embodiment of the present invention, a near real-time frequency hopper jamming system. Once armed for jamming, the system first receives and instantaneously processes a wide bandwidth of RF spectrum. The invention will then detect short duration signals such as frequency hopping signals and burst transmissions. Such suddenly appearing signals are then automatically evaluated and then automatically jammed by this invention. The combination between the automatic detection of sudden, short duration signals, and the intelligent evaluation and jamming of such signals, was unique until the disclosure of that prior jamming system and method of the parent application to this continuation-in-part.
The system 80 is implemented in hardware and preset by software programming. The system 80 uses a device 12 that is a wideband front-end downconverter (i.e. a radio receiver tuner) that outputs a wideband intermediate frequency (IF). Thus all the signal information contained within the bandwidth of the IF filter can be analyzed instantly. The resulting IF output may contain one or many short duration communication signals. The front-end section of the System utilizes portions of wideband detection technology described by the patent application entitled: “Method And Apparatus For The Intelligent And Automatic Gathering Of Sudden Short Duration Communications Signals” (U.S. patent application Ser. No. 10/829,858). The next sections contain the selection logic by which it is automatically determined whether or not received signals should be jammed. There are various programmable criteria. For example there is a section that determines priority for jamming, as well as a lookup table for jammer programming. The cycle generator section 30 regulates the user configurable System timing. The final section of the invention executes the jamming frequency generation and output (as determined by the previous sections), which must also occur extremely quickly. All of these processes occur in near real time.
Operation
As a high level description, the invention described herein first basically has the hardware and method required to capture high speed frequency hopping transmissions. Then the frequencies of those detected signals are passed along through a series of decision modules to determine whether or not it should be jammed. The signal logic modules of the jammer take those results and strip out the signals that are not of interest. What remains are the signals that should be jammed and are subsequently passed on to the invention's signal generation circuitry for jammer output. All of this occurs automatically and without CPU intervention, everything can be done in hardware.
To jam high speed frequency hoppers requires equipment that has extremely fast and especially precise timing. The invention of that prior patent application uses such a concept and implements it with a hardware module called the cycle generator 30. The jammer system has all of the timing regulated and coordinated by the cycle generator 30, which has its' timers pre-programmed in the Setup Mode. After the Setup Mode is complete and the system is properly “armed” (please see below a discussion of Modes), the operator can begin the Attack Loop Mode by initiating the cycle generator into action which will in turn make the entire jammer system operate autonomously until the jammer is manually turned off by the user, or the attack timer expires.
During operations, a converter device 12 is first tuned to a region of the RF spectrum where the enemy frequency-hopping signals are expected to be. A PIN diode switch 10 is placed on the input to this converter device 12. At first, the PIN diode switch is naturally in the closed, or connected, position to allow signals to pass through and into the converter. Incidently, during the Attack Loop Mode of operations, this switch 10 is commanded open by the cycle generator 30 to protect the converter's 12 front-end amplifiers (so-called “blanking” of the front-end) when the System 80 is transmitting at high power RF.
The converter 12 acts as a down-converter device to properly shift the received spectrum into a usable IF range. The wide band analog IF output is then fed through a bandpass filter and then the filtered analog IF is fed directly into the analog-to-digital (A/D) conversion component 14 for digitization. The digitized IF data is then fed to a hardware logic component that performs fast Fourier transformations (FFT). This FFT module (such as an FPGA device) performs various DSP algorithms. But the FFT function is not initiated until the cycle generator sends it the “Start FFT” command.
When the operator is ready, or upon order from military command, the cycle generator 30 is initiated. This cycle generator then sends the Start FFT command to the FFT module and the “detection timer” begins. This gets the entire jammer going and listening for enemy signals (as described above). The FFT module performs the FFT's and transforms the incoming digitized IF (which is in the time domain) to the frequency domain. The FFT length can be 1024 points or more. The output of eatch FFT bin is digital I and Q data. The I and Q data is combined by a magnitude algorithm which takes the square root of the sum of the individual squared values of I and Q. The result is the normalized amplitude of the I/Q, which is the processed spectrum. Thus the spectrum data is completely in the mathematical real domain, without any imaginary components. The amplitude of the “bins” of the FFT correspond to the signal energy detected in each FFT sample over the total IF bandwidth. Each FFT bin value thus contains the amplitude value over a narrow frequency band.
The output of the FFT module is an FFT bin array of information (so-called FFT frame) that is then fed to another hardware logic component 18 (such as an FPGA) that determines if the incoming spectrums contain signals on frequencies that were preprogrammed to not be jammed. The module takes in the incoming FFT bins and excludes those bins that the user does not want to jam. These “lockouts” are bins that translate to no-jam frequencies of friendly or coalition forces (which are provided during the System pre-programming phase—Setup Mode). These are “fixed” lockouts; there are also “real-time” lockouts that may be applied as a function of the hopping pattern that friendly and coalition forces' radios are expected to use at that current time. These real-time lockouts protect the so-called “fill of the day” hopping pattern so that they are not jammed. The result is that the FFT frame that is allowed to pass will only contain present signals that were not designated to be locked out by the jammer. These lockouts can alternatively be done at a later stage without affecting the function of the jammer. The hardware logic then takes the remaining FFT bins and performs various peak detection algorithms 20 (such as two, three or five-point methods) on the set. This algorithm 20 continually takes in new FFT bins all the time and updates the calculated output values for each bin. This is done to improve the overall signal-to-noise (S/N) of the system to receive the new signals. This process all occurs within the Setup Mode programmable “Detection Time” period. The longer detection time used, the higher signal to noise ratio. After the detection period has expired, the cycle generator sends a “Stop FFT” command to the FFT module 16. The FFT module 16 will finish its current FFT frame generation and then pass them on to the following modules. But after that, there will be no more FFT frame generation until the very next attack cycle begins.
The FFT module 16 then sends a command to the peak detection algorithm module 20 to wait for the final FFT frame to arrive, and then to release the final values of the FFT bins and send them to the signal evaluation algorithm module 22.
This algorithm 22 makes a determination if a signal is present in each of the bins or not by using the user provided “window amplitude threshold settings” as a rule set. The window threshold settings are configurable upper and lower amplitude bounds for a signal to be declared present. These values are input during the Setup Mode phase (described below). If a signal does not land within the configurable window threshold setting, then the signal is not jammed. The idea is to not jam signals that have too high a signal strength, since they are typically considered to be from nearby (friendly) forces. If the signal is too low, then it does not meet the minimum signal threshold requirements. This avoids jamming noise spikes. If one or several targets are identified as suitable to jam by the signal evaluation logic 22, those are then sent to the priority logic algorithm module 24 of the hardware.
This priority logic algorithm module 24 decides which one of the signals (or which group of signals) will be jammed. Priority rules can be either hard coded, or configured by the user during the Setup Mode phase. Some signals might be pre-programmed to have higher importance than others for example. After a determination of which signals to jam has been made, those frequencies are matched to the proper Direct Digital Synthesizer (DDS) programming data that is determined from a lookup table.
Next, the information is then sent, along with the DDS programming data, to a Direct Digital Synthesizer module 26 (DDS) that in turn outputs the required jamming frequency, or frequencies. To program DDS chips may require many CPU operations that would take precious milliseconds to accomplish. Thus, one way to effectively program the DDS chips with enough speed is to have a pre-programmed lookup table of every single DDS programming array of bits that are matched to each and every frequency bin of the final FFT frame that comes out of the priority select logic module 24. Thus, there is no CPU intervention required to program the DDS chips. The DDS frequencies are then generated automatically in FPGA hardware.
The output of the DDS module 26 are jammer signals. The jammer signals are then sent to an upconverter stage that contains a oscillator 28, mixer 32 and output filter. The proper final jamming frequency is then output from the System to the external high power amplifier 34 (PA) for long range transmission.
Operation Modes
As mentioned earlier, this system 80 has two major operational modes, a Setup Mode, and the Attack Loop Mode. In the Setup Mode, the operator inputs several parameters to “arm” the System properly with the right information in order to perform fast reactive jamming. For example, the System is programmed with which specific frequencies are NOT to be jammed (i.e. lockouts). This is important since friendly communications should not be attacked during a jamming cycle. The Setup Mode has several major parameters to be input prior to allowing it to go into Attack Mode.
The first Setup Mode parameter involves the tuning of the wideband downconverter 12. This is necessary so the system can “listen” in the right RF spectrum range where enemy frequency-hopping signals are expected to be.
The second Setup Mode parameter involves the programming of the memory logic, window threshold settings, and the priority selection criterion 24. In addition, the DDS setting tables are pre-loaded. The lockout memory logic 18 contains the frequencies that are “locked out” so the System will not jam those. The jammer also contains the priority selection algorithms 22 that are used to evaluate the amplitudes of the FFT bins to see whether or not there is a signal present. The DDS tables are pre-calculated arrays of DDS programming information. Each element in the array corresponds to a different frequency bin within the processed spectrum. For example, if a signal is detected in bin #45, then that frequency has a proper DDS setting in order to jam that frequency. The DDS table thus contains the proper DDS programming information in order to quickly set the DDS chipset 26 to any frequency that a signal appears on within the IF spectrum. When a jamming signal is identified, the hardware logic 24 does a lookup on the table and feeds the correct DDS programming to the DDS 26 itself. And as mentioned, this in turn automatically makes the DDS output the proper frequency.
This method is employed because to program a DDS chipset 26 to output a frequency would take several cycles for a CPU to execute. And those cycles are too long for jamming a fast frequency hopper. Thus, pre-programming fast memory logic to output the correct DDS input which corresponds to the correct jamming signal frequency is there to make the Attack Loop time short enough to—engage fast frequency hopping signals.
The third Setup Mode parameter involves the tuning of the upconverter oscillator 30. Since the DDS chipset 26 may not have enough frequency range to do full frequency coverage, it may be necessary to do the upconversion in a separate stage. But to prepare it for the Attack Loop Mode, the upconverter oscillator 30 is set in advance so the upconverter will cover the targeted frequency range.
The fourth Setup mode parameters that need to be set involve programming the cycle generator 30. The cycle generator 30 is a set of registers that will command the System during the Attack Loop Mode. This is necessary to orchestrate the series of events in the right times, needed to successfully jam a received signal. One parameter that needs to be set is the detection time. This time is how long the System listens for incoming signals and processes the FFT'S. Another parameter that needs to be set is the jamming transmission ON time, or “jamming dwell time”. It is necessary for the System to jam for only a certain dwell time, after which the System needs to see if any of the attacked frequencies have hopped to a new location. And the final parameter that needs to be input is the attack time. The attack time is how long the invention should remain in Attack Loop Mode, before stopping and going back into Setup Mode. A manual stop can also be done at any time.
The cycle generator 30 also provides the physical signaling controls to open and close the input PIN diode switch 10 to protect the front-end downconverter 12. This is done to limit the jammer signal power into the downconverter 12 when the PA 34 is transmitting. The cycle generator 30 also sends the signal commands at the proper microsecond timing in a jamming cycle to turn on the PA 34 at the beginning of the jamming dwell period. It also turns off the PA 34 at the end of the jamming dwell period at the proper microsecond timing. After turning off the PA 34, the cycle generator 30 also opens the PIN diode switch 10 and resets the detection timer. Thus, a whole new jamming cycle can begin. This process loops over and over until the user manually cancels Attack Loop Mode, or until the system attack timer expires.
After all these parameters are set, the operator then commands the invention 80 into the Attack Loop Mode when ready, or ordered to by Military Command. In this mode, the system 80 simply monitors the RF spectrum that it was assigned to. And if any short duration frequency-hopping signal arrives within that range, the system 80 will automatically send out a jamming signal in near real time. As mentioned, the operation continues for a user programmable period of time (attack time), or until the operator manually cancels the Attack Loop Mode and brings the System back into Setup Mode.
What is needed is a modification to this extremely responsive surgical reactive jammer that enables the jammer to more accurately determine the target frequency of the jammer than this prior system can.